Hydrogen fuel cells generate power through a reaction between hydrogen gas as the fuel gas and oxygen gas as the oxidization gas. Operation of the fuel cell is started by supplying the hydrogen gas to a hydrogen electrode (anode) of the fuel cell while supplying the oxygen gas to an oxygen electrode (cathode), and stopped by stopping the supply of the hydrogen gas and oxygen gas to the fuel cell. When the operation of the fuel cell is stopped with fuel gas and oxidization gas still remaining in the fuel cell, a pressure difference causes the residual gas to move through the electrolyte between the hydrogen electrode and the oxygen electrode, thereby generating a chemical reaction which increases the temperature to a level that may damage the electrolyte (i.e., a chemical short). Because of this, residual gas inside the fuel cell is scavenged by supplying an inert gas inside the fuel cell while it is stopped. Also, as disclosed in JP-A-4-4570, when the fuel cell is in a standby state in which operation thereof is stopped for a relatively short period of time during which power is not supplied from the fuel cell to a load, small amounts of hydrogen gas and oxygen gas continue to be supplied to the fuel cell during this time in order to prevent residual gas from moving through the electrolyte in the fuel cell.
However, when scavenging residual gas using an inert gas when the fuel cell is stopped, not only must the necessary amount of inert gas be stored beforehand, but a scavenging mechanism for scavenging and a tank must be provided separately from the fuel gas/oxidization gas supply system, which is undesirable from the viewpoint of reducing the size as well as the manufacturing costs of the fuel cell. Furthermore, with a construction in which small amounts of hydrogen gas and oxygen gas are continually supplied to the fuel cell and the gases are circulated within the fuel cell even when it is in the standby state, the fuel consumption efficiency decreases due to the fact that auxiliary devices such as a compressor are operating even when power is not being supplied to a load. This is undesirable for obtaining an on-board fuel cell and the like.